Engine exhaust often contains incomplete combustion compounds such as hydrocarbons (HC), carbon monoxide (CO), and NOx. These compounds must be removed for air pollution control and to meet various government regulations. There are various catalysts and systems used for the treatment of such exhaust gas. For example, three-way catalysts (TWC), close-coupled catalysts, filters (which may be catalyzed) have been utilized to address challenging emission problems for different engines and fuel configuration. Most of these catalysts or combined catalysts systems are based on the precious metals (also known as “platinum group metals” or “PGM”) of platinum (Pt), palladium (Pd), rhodium (Rh) and iridium (Ir). Although these precious metal catalysts are effective for mobile emission control and have been commercialized in industry, the extremely high cost of these precious metals remains to be a critical factor for wide spread applications of these catalysts.
Base metals are abundant and much cheaper than precious metals. Several attempts have been made to develop catalysts based on base metals for emission control. However, these base metal catalysts often do not have sufficient activity for saturated HC and NOx conversions and thermal stability to meet regulation requirements for mobile emission control.
Other attempts have been made to incorporate base metals into a precious metal catalyst material. However, such incorporation of base metal into a platinum group metal-based three-way catalyst washcoat or formulation resulted in a poisoning effect of the base metal, which results in degradation of PGM three-way catalyst performance.
Other base metal formulations were intended for HC or sulfur trapping purposes and are generally not efficient TWC catalyst. Although these examples showed some benefits in reduction of sulfur compounds, PGM are often poisoned by base-metal particularly after high temperature aging. Therefore, the addition of base-metals to the PGM formulations has not been very successful to significantly reduce the PGM loading thus the cost of the TWC catalysts.
Therefore, there is a need of alternative, cheaper catalyst materials that are also effective for the removal of hydrocarbons, CO and NOx compounds from mobile emission sources and meet increasingly stringent regulations.